Tang and coworkers first reported on high-performance organic light-emitting devices (OLEDs) in 1987 (Tang, C. W.; et al. Appl. Phys. Lett. 51, 913 (1987)). Their discovery was based on employing a multilayer structure containing an emitting layer and a hole transport layer of a suitable organic substrate. Alq3 (q=deprotonated 8-hydroxyquinolinyl) was chosen as the emitting material and proven to be of high-performance because (1) it can form uniform thin films under 1000 Å using vacuum deposition, (2) it is a good charge carrier and (3) it exhibits strong fluorescence. Since then, there has been a flourish of research on OLEDs and materials used in these devices. Indeed, nearly every large chemical company in the world with optoelectronic interests has demonstrated some level of interest in OLEDs. Clearly, OLED technology is heading directly and rapidly into the marketplace. The attractiveness of OLEDs as it challenges traditional technologies such as cathode ray tubes (CRTs), liquid crystal displays (LCDs) and plasma displays is based on many features and advantages, including:                Low operating voltage,        Thin, monolithic structure,        Emits, rather than modulates light,        Good luminous efficiency,        Full color potential, and        High contrast and resolution.        
OLED is a device built with organic semiconductors from which visible light can be emitted upon electrical stimulation. The basic heterostructure of an OLED is described in FIG. 1.
The layers may be formed by evaporation, spin-casting or chemical self-assembly. The thickness ranges from a few monolayers (self-assembled films) to about 1000 to 2000 Å. Such devices whose structure is based on the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission, namely, the radiative recombination of a trapped charge. Under a DC bias, electrons are injected from a cathode (usually Ca, Al, Mg—Ag) and holes are injected from an anode (usually transparent indium tin oxide (ITO)) into the organic materials, where they travel in the applied field across the electron transporting layer (ETL) and the hole transporting layer (HTL) respectively until they meet, preferably on molecules in the emitting layer, and form a luminescent excited state (Frenkel exciton) which, under certain conditions, experiences radiative decay to give visible light. The electroluminescent material may be present in a separate emitting layer between the ETL and the HTL in what is referred as a multi-layer heterostructure. In some cases, buffer layers and/or other functional layers are also incorporated to improve the performance of the device. Alternatively, those OLEDs in which the electroluminescent emitters are the same materials that function either as the ETL or HTL are referred to as single-layer heterostructures.
In addition to emissive materials that are present as the predominant component in the charge carrier layers (HTL or ETL), other efficient luminescent material(s) may be present in relatively low concentrations as a dopant in these layers to realize color tuning and efficiency improvement. Whenever a dopant is present, the predominant material in the charge carrier layer may be referred to as a host. Ideally, materials that are present as hosts and dopant are matched so as to have a high level of energy transfer from the host to the dopant, and to yield emission with a relatively narrow band centered near selected spectral region with high-efficiency and high-brightness.
While fluorescent emitters with high luminescence efficiencies have been extensively applied as dopant in OLEDs, phosphorescent emitters have been neglected in this domain. However, the quantum efficiency of an electrofluorescence device is limited by the low theoretical ratio of singlet exciton (25%) compared to triplet exciton (75%) upon electron-hole recombination from electrical excitation. In contrast, when phosphorescent emitters are employed, the potentially high energy/electron transfer from the hosts to the phosphorescent emitters may result in significantly superior electroluminescent efficiency (Baldo, M. A.; et al. Nature 395, 151 (1998) and Ma, Y. G.; et al. Synth. Met. 94, 245 (1998)). Several phosphorescent OLED systems have been fabricated and have indeed proven to be of relative high-efficiency and high-brightness.
It is desirable for OLEDs to be fabricated using materials that provide electrophosphorescent emission corresponding to one of the three primary colors, i.e., red, green and blue so that they may be used as a component layer in full-color display devices. It is also desirable that such materials are capable of being deposited as thin films using vacuum deposition techniques, which has been prove to be a common method for high-performance OLED fabrication, so that the thickness of the emitting layer can be precisely controlled.
Presently, the highest efficiencies and brightness have been obtained with green electrophosphorescent devices (15.4±0.2% for external quantum efficiency and almost 100% for internal efficiency, 105 Cd/m2 for maximum luminance) using Ir(ppy)3 (ppy=deprotonated 2-phenylpyridine) as emitter (Adachi, C.; et al. Appl. Phys. Lett. 77, 904 (2000)). An OLED emitting saturated red light based on the electrophosphorescent dopant Pt(OEP) (H2OEP=octaethylporphyrin) has also been published and patented (Burrows, P.; et al. U.S. Pat. No. 6,048,630) but the maximum luminance is only around 500 Cd m−2. A relevant patent is the use of the cyclometalated platinum(II) complex Pt(thpy)2 (thpy=deprotonated 2-(2-thioenyl)pyridine) as dopant and PVK (poly(N-vinyl)carbazole) as host in a orange OLED (Lamansky, S.; et al. WO Pat. No. 00/57676). However, the Pt(II) complex used by the inventors was not stable for sublimation or vacuum deposition, thus a spin-casting method was applied, which led to higher driving voltages, quantum efficiency of 0.11% and luminance of 100 Cd/m2 were obtained at 22 V.